I just checked the anode in my gas water heater which is now about 6
years old. The rod was about 1/3 gone. I put it back in and will
check again in about 2 years. I think it's very logical that a $35
rod replacement could prolong the life of the tank more than enough to
justify the cost. As a previous poster pointed out, sacraficial anodes
are used routinely on boats and work. The tank manufacturers
obviously believe it too, as they put them in to begin with. If you
have a tank in conditions where the anode errodes faster, then
replacing it makes sense to me.